Modeling the Role of Root Exudation in Critical Zone Nutrient Dynamics.

The multifaceted interface of plant roots, microbes, water, and soil can be considered a critical zone within the Critical Zone as it is host to many important dynamically linked processes, including the promotion of nutrient cycling through absorption and rhizodeposition, interaction and feedbacks with microorganisms and fungi, root‐facilitated hydraulic redistribution, and soil carbon dynamics. Such important processes in the Critical Zone have not been fully characterized and modeled in an ecohydrologic framework linking above‐ground natural and/or anthropogenic processes to below‐ground biogeochemical cycling. Specifically, the relation between root exudates and nutrient cycling remains an open challenge. Here we present the model REWT (Root Exudation in Watershed‐scale Transport) to demonstrate the systematic modeling of root exudation in an interconnected ecohydrologic framework. REWT incorporates an explicit dynamic root exudation transport model, nutrient absorption, and coupled microbial processes within the framework of a validated ecohydrologic model. Model simulations demonstrate the influence of root exudation of glucose, a polysaccharide that serves as fuel for microbes, and flavonoids, which can act as a biological nitrification inhibitor on microbial processes linked to soil carbon and nitrogen cycling. To demonstrate the capabilities of this theoretical framework, we parameterize REWT for corn and soybean crops in the Midwestern United States, and simulations indicate that rates of nitrification and respiration were substantially altered compared to model simulations in which root exudation was not explicitly included. This work demonstrates the importance of systematically incorporating root exudates into hydrobiogeochemical models and can serve to inform experimental design for active root zone processes. Plain Language Summary: Root exudation is the highly complex and variable process by which roots release carbon and other nutrients into the surrounding soil to influence microbial activity and biogeochemical conditions to their benefit. It is a process common to all types of plants and trees and has important implications for nutrient cycling and soil health. Here we develop a model, REWT (Root Exudation in Watershed‐scale Transport), which couples exudation with moisture transport and microbial transformation of nutrients in agricultural and natural soils in an effort to more accurately capture dynamic biogeochemical processes in the Critical Zone. We present REWT simulations for corn and soybean crops in the Midwestern United States. These demonstrate that explicitly incorporating root exudation into hydrobiogeochemical models can result in substantial increases or decreases of microbial biomass, rate of nitrification, and total nitrate leached by the soil system, in this case on the order of ±4%, up to 100% decrease, and up to 16% decrease, respectively. REWT simulations are not only promising for future work related to plant breeding and modeling of biogeochemical cycling in natural and agroecosystems; they also shed light on the need for more empirical work related to rhizosphere processes, particularly the measurement of root exudation rates. Key Points: Roots influence subsurface biogeochemical dynamics and mediate microbial dynamics and associated geochemical feedbacks via exudationRoot exudates both enhance microbial activity, such as through glucose production, and inhibit it, such as through production of flavonoidsHere, the highest sensitivity of exudation's effect on nutrient dynamics results from nitrification inhibition through flavonoid exudation [ABSTRACT FROM AUTHOR]